In many industrial applications there is a need to monitor the movement of a movable part. Such monitoring requires the delivery of an encoder output signal indicative of the movement, or indicative of the position of the movable part.
It is conventional to provide an encoder apparatus in which the encoder output signal is generated in dependence on a detected magnetic field. A magnetic encoder may include a magnetic scale that can be installed on a rotatable shaft of a machine. U.S. Patent Application Publication No. 2010/0207617 describes an incremental encoder for detection of the movement of a shaft including a scale having a series of spaced magnetic marks mounted on the circumference of the shaft. A read head is mounted on another machine part so as to pass along the scale, the read head including a first set of detectors 16-1 for sensing the scale marks, and generating a pulsed signal in response to rotational movement of the shaft and scale in relation to the read head. In order to be able to generate an encoder output signal indicating not only incremental movement of the shaft, but also providing a position output signal, a counter is used for counting the pulses of the pulsed signal, the counter value being indicative of the position. In order for the counter value to reflect the position of the shaft within a single revolution, it is conventional to provide the magnetic scale with a magnetic reference marker. In this manner, the reference marker is used for generating a reference signal once per shaft revolution in response to the magnetic reference marker passing a separate second set of reference sensors 16-2 provided in the read head, as described in U.S. Patent Application Publication No. 2010/0207617. Providing a separate reference marker next to the scale requires additional space, which may not be available, as pointed out in U.S. Patent Application Publication No. 2010/0207617. To address this problem, U.S. Patent Application Publication No. 2010/0207617 describes providing the reference mark by further magnetizing the scale in a region of one of the existing pole pitches so as to produce a reference signal. The further magnetization extends across only a part of the width of the scale, covering less than half of the width, so that the incremental signals may be read undisturbed, according to U.S. Patent Application Publication No. 2010/0207617.